Injection of carbon dioxide for tertiary enhanced recovery of oil from hydrocarbon reservoirs has been known and used worldwide since at least the 1980s. In particular, there are numerous carbon dioxide tertiary enhanced recovery projects in operation in the Permian Basin oil fields of west Texas. General literature about the conduct of such projects is well known and available from such sources as the Journal of Petroleum Technology and in papers published by the Society of Petroleum Engineers. Generally, in a carbon dioxide tertiary enhanced recovery project, the carbon dioxide is injected into a hydrocarbon reservoir via injection wells penetrating the producing formation. Oil, hydrocarbon gas, and water are produced through offsetting production wells.
Parrish, U.S. Pat. No. 4,344,486, incorporated by reference, teaches that the effectiveness of carbon dioxide as an aid to oil recovery is dependent on its miscibility pressure. As the carbon dioxide flows through a reservoir at an underground pressure above about 1,000 psi and a temperature of about 100 to 150° F., the carbon dioxide becomes partially miscible with the oil and helps push the oil toward the wellbore. The miscibility of the carbon dioxide with oil is dependent upon many factors including carbon dioxide purity, oil type, reservoir pressure, and reservoir temperature. The oil-carbon dioxide miscibility can be negatively affected by contaminants such as nitrogen, oxygen, oxides of nitrogen, carbon monoxide and methane. Parrish, U.S. Pat. No. 4,344,486, discloses that it is desirable for the carbon dioxide injection stream used in enhanced oil recovery to be substantially free from contaminants.
After carbon dioxide has been injected into the producing formation, the carbon dioxide will move through the producing formation driving a “flood front” of oil ahead of it toward the producing well. Ultimately, some of the carbon dioxide will reach the producing well and carbon dioxide will be produced in the production well together with the oil and hydrocarbon gases. The produced oil and gas mixture must be separated into its components.
At a primary field separation facility, oil is removed, treated, and sold. Free water, not entrained in the gas, is separated and disposed of or re-injected into the reservoir. The gaseous phase of the fluid stream is separated and sent from the primary field separation facilities to a central gas processing facility. In smaller fields, one gas processing facility may serve several fields. In a carbon dioxide enhanced recovery project, the produced gas will be a mixture of hydrocarbon gases and carbon dioxide. Additionally, some impurities such as hydrogen sulfide may also be present.
Combustion gas turbines capable of using low Btu gas as gas turbine fuel are well known in the art. Integrated gasification combined cycle (IGCC) systems have been used successfully to burn low caloric value (LCV) fuel. IGCC is a process in which a LCV fuel such as coal, petroleum coke, orimulsion, biomass or municipal waste may be converted to a low heating value synthetic fuel, which is used as the primary fuel in a gas turbine. Synthetic fuel has a heating value of about 125 Btu/scf to 350 Btu/scf. Typical natural gas has methane as its primary component and has a heating value of about 1,000 Btu/scf. The synthetic gas heating value and components may vary widely from one application to another and are highly dependent on the particular process producing the gas, the oxidant used, and the process feed stock. Further information on IGCC and flammability as a function of caloric value is discussed in a technical paper authored by R. D. Brdar and R. M. Jones, titled GE IGCC Technology and Experience with Advanced Gas Turbines, and is incorporated by reference.
Zapadinski, U.S. Patent Publication 2004/0154793 A1, incorporated by reference, discloses a method and system of developing a hydrocarbon reservoir wherein hydrocarbon gas from the field is combusted with air as an oxidant in a gas engine and the exhaust gas resulting from the combustion is compressed and then injected into the hydrocarbon reservoir. The exhaust gas of the system taught by Zapadinski includes a high percentage of nitrogen and nitrogen oxides in addition to carbon dioxide. The nitrogen comes from using air as the oxidant, since air contains about 79% nitrogen. At a given injection pressure, injection of carbon dioxide containing nitrogen or nitrogen oxides into a hydrocarbon reservoir is less efficient in the enhanced recovery process than use of pure carbon dioxide due to the negative effects of nitrogen on the miscibility of the injected gas with the oil in the reservoir. Kovarik F S, “A Minimum Miscibility Pressure Study Using Impure CO2 and West Texas Oil Systems: Data Base, Correlations, and Compositional Simulation,” Society of Petroleum Engineers Production Technology Symposium, November 1985, incorporated by reference.
In both an IGCC and a typical combined cycle system, the gas turbine compressor uses atmospheric air as the source of oxygen for combustion. In such a system, air is the working fluid in the system and the turbine exhaust gas is released to the atmosphere after heat capture in a heat exchanger or heat recovery steam generator. Alternatively, in a semi-closed combined cycle, the turbine exhaust gas is recirculated back to the inlet compressor. Although a semi-closed combined cycle using atmospheric air as the oxygen source will result in an exhaust gas and overall working fluid enriched in carbon dioxide, the working fluid will still contain a large nitrogen component, making the carbon dioxide containing slipstream much less than optimal composition for use in the enhanced recovery process. Finally, substantially pure oxygen may be used as the oxidant in a semi-closed combined cycle. By using substantially pure oxygen instead of air for combustion, the purity of the carbon dioxide in the exhaust gas stream and the overall working fluid is much higher. The literature contains many articles that discuss various aspects of the semi-closed combined cycle process and the change in working fluid from air to carbon dioxide, and its effect on the performance of the inlet compressor and gas turbine due to the difference in fluid properties. Roberts S K, Sjolander S A, 2002, “Semi-Closed Cycle O2/CO2 Combustion Gas Turbines: Influence of Fluid Properties on the Aerodynamic Performance of the Turbomachinery. ” ASME GT-2002-30410. Proceedings of ASME TURBO EXPO 2002, Amsterdam, The Netherlands, Jun. 3-6, 2002, incorporated by reference. According to Roberts and Sjolander, two fluid properties that should be considered when switching from air to a carbon dioxide working fluid include the ratio of specific heats (γ) and the gas specific constant (R). At any given temperature, the carbon dioxide working fluid has a lower ratio of specific heats, lower gas specific constant, and higher density as compared to the air working fluid. The ratio of specific heats for carbon dioxide is approximately 1.28 at 300 K, the ratio for air is approximately 1.40 at 300 K and the ratio for water vapor (a product of combustion) is 1.14 at 300 K. Similarly, the gas specific constant differs significantly between carbon dioxide (188.9 J/kg-K), air (288.2 J/kg-K) and water vapor (461.5 J/kg-K).
Another journal article describes how the ratio of specific heats (γ) and the gas specific constant (R) are used to calculate the turbo machinery non-dimensional mass flow (πM) and non-dimensional speed (πN) parameters, which also need to be considered when changing the working fluid from air to carbon dioxide. Jackson A J B, Neto A C, Whellens, M W. 2000. “Gas Turbine Performance Using Carbon Dioxide as Working Fluid in Closed Cycle Operation.” ASME 2000-GT-153. ASME TURBOEXPO 2000, Munich, Germany, May 8-11, 2000, incorporated by reference. The large difference between the ratio of specific heats (γ) and gas specific constant (R) for carbon dioxide and air affects the turbo machinery non-dimensional mass flow and non-dimensional speed parameters and this presents a challenge for using existing turbo machinery equipment for a carbon dioxide working fluid.